A young family are devastated after a major house fire wrecked their suburban home this afternoon.

Wellington firefighters described a difficult challenge battling the blaze but said it was fortunate nobody was hurt in the Normandale Rd fire in Lower Hutt.

Two hours after the blaze broke out, the roof of the house was still smouldering, the property off-limits and the smell of burned wood thick in the air.

Neighbour Jess Judd said she felt terrible for the young family.

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"I think they're going to try to gather everything they can gather," she said. "It was a young family - a family with two small kids."

She said the family were away from the double-storey home when the fire broke out this afternoon but a firefighter found a package near the front door with a resident's name on it and called them.

The Fire Service was this evening looking after the visibly devastated family at a mobile command unit a few sections away from the burned house.

Fire Service assistant area commanders Henry Stechman and Matthew Abel said the fire was mostly concentrated in a large horizontal gap between the two floors.

At the scene, Mr Stechman said about 20 Fire Service staff were on site at about 6.30pm.

"It's one of the most difficult these guys have had to deal with."

Narrow, winding Normandale Rd make access difficult for fire engines but the source of the fire inside the house complicated efforts to extinguish it.

"It appeared to be within the floor space and up the walls," Mr Stechman said. Glass also started falling from windows around the firefighters.

"We didn't know how stable the floor was," Mr Abel said.

The crews withdrew and went into a "defensive" mode to isolate the fire.

Some grass and hedging at a property next door caught fire but no other houses were burnt.

Mr Abel said it was "very, very fortunate" nobody was hurt.

He said the fire appeared to start at the back of the house, which was a timber building with a stucco-type veneer.

Ms Judd understood there had been builders at the house earlier but wasn't sure how the fire started.

The Fire Service was investigating and Mr Abel and Mr Stechman said it was too soon to tell what started the fire.